Ultra-high frequency band separating filters



Dec. 30, 1958 G. R. P. MARIE 2,866,595

ULTRA-HIGH FREQUENCY BAND SEPARATING FILTERS Filed Feb. 26, 1957 5 Sheets-Sheet 1 Dec. 30, 1958 G. R. P. MARIE 2,866,595

ULTRA-HIGH FREQUENCY BAND SEPARATING FILTERS Filed Feb. 26. 1957 5 Sheets-Sheet 2 fly. 4

Dec. 30, 1958 G. R. P. MARIE 2,856,595

ULTRA-HIGH FREQUENCY BAND SEPARATING-FILTERS Filed Feb. 26, 1957 5 Sheets-Sheet 3 United States Patent ULTRA-HIGH FREQUENCY BAND SEPARATING FILTERS Georges Robert Pierre Mari, Paris, France Application February 26 1957, Serial No. 642,517

Claims priority, application France April 28, 1956 7 Claims. (Cl. 333-73) The present invention relates to branching filters for use with a wave guide for the separation of waves, the frequencies of which cover wide frequency bands in. the range of centimeter waves.

These frequency bands can, for example, be those of channel groups of carrier wave systems. For instance, a filter according to the invention can separate a first frequency band, hereinafter called lower band and covering the range of 3,000 to 5,000 megacycles per second from a second frequency band, hereinafter called upper band and covering the range of 7,000 to 10,000 megacycles per second.

in order to simplify the following description, the lower limit of the lower band and the upper limit of the upper band will be called the outer frequency limits of the frequency bands to be separated. On the other hand, the upper limit of the lower band and the lower limit of the upper band will be called the inner frequency limits.

The band separating filters, which are provided in accordance with the objects of the invention, may be of a type in which the two signal bands are received from anysource of waves at the input of a main wave guide of rectangular cross-section propagating a TE mode wave and in which the signals of the lower frequency band are further transmitted in the same form towards the output of this main guide, while the signals of the upper frequency band are derived towards a secondary guide also of rectangular cross-section and coupled to the said main guide along a certain length thereof by slots provided in the lateral walls of the two guides.

Such separating filters and their theory have already been described in detail, for instance, in a paper by S. E. Miller, publishing in the review Bell System Technical Journal, May 1954, pp. 661-670. Filters forthe same purpose are also described in a paper by J. R. Pierce,

published in the review Proceedings of the Institute of Radio Engineers, February 1949, pp. 152-155, as well as in the U. S. Patent 2,626,900 in the name of J. R. Pierce. The operation ofthe' filters described in the'last two mentioned works is based on the resonance-of short lengths of an auxiliary wave guide interconnecting the slots in the walls of the twoguides. The latter filters, therefore, allow derivation of such signals from the main guide only if their frequency band is relatively narrow.

The filters described in the above-mentioned paper of i the Bell System Technical Journal allow, on the other hand, derivation of signals covering wide frequency bands from the main guide, but they are very bulky, as, in order to avoid a perturbation of the signals of the lower fre quency band which should continue to propagate in the mainguide and also to avoid formation of parasitic propagation modes,,the coupling slots must be of progressively varying sizes and distributed along a great length of the guide.

The filters provided in. accordance with the invention obviate this draw-back by 'a particular choice of so-called I guides of rectangular cross-section.

2 coupling elements inserted between the slots in" the respective walls of both guides.

In accordance with one embodiment of the present invention, there is provided a band separating filter adapted to be inserted in a main wave guide of rectangular crosssection in which two signal groups, the frequencies of which cover respectively a lower frequency band and an upper frequency band, separated by a frequency-interval, propagate according to the TE mode. The filter comprises a length of the main guide at the input of which the two signal groups are applied and at the output of which the signals of said lower band are received,

while the signals of the upper band are directed towards at least one secondary wave of rectangular cross-section and having its axis parallel to that of the main guide and coupled to the latter guide by a first series of slots distributed along said length of the main guide and provided in its walls and by a second series of slots pro- .vided in thew 'of the upper frequency band. The filter is further characterized in that the dimensions of thecross-sections of the coupling guides vary progressively along the length of the main guide, so that their characteristic impedances have a maximum value in the middle and a minimum value at the; ends of the length.

Filters provided by the invention thus consist of one or more directional couplers-withmultiple couplingscomposed of main and secondary guides and of coupling These couplers differ from those known in the art in that the cut-off frequency of the coupling guides is comprised between the inner limits of the frequency bands to be separated. They also differ from previously known couplersin the provision of certain devices for the connection of the secondary guides deriving the energy of'the waves of the upper frequency band. Owing to their symmetry, these devices avoid the formation of higher propagation modes which would otherwise be likely to appear. 7

According to its simplest method of construction, a filter of the invention contains only one secondary" guide coupled to the main guide, as explained above.

According to this first method; the filter of the invention consists of amain guide and ofasecondary guide with parallel axes. These guides having equal cut-off frequencies lower than the l'owestfrequency of the waves used and are coupled by a plurality of rectangular crosssection coupling guides, the axes of which are parallel between themselves and perpendicular to those of the two" guides which they couple. The cut off fre'quencies of the, coupling guides are all equal between themselves with a common value comprised between the inner limits of the frequencybands to be separated. As a result, the waves of the upper band are transferred from the main guide across the coupling guide to thesecondary guide, while the waves of' the lower band remain in. the main guide at'the input of which they were applied; The longer sides of the cross-sections of the coupling guides are thus perpendicular to the direction of'the longitudinal currents in the walls of both the main and secondary guides. i

The coupling guidesv dov not have. the same. characteristic impedance. The value of this impedance is lowat-i the extremities of the coupling length; on the other hand, it

assesses is high in the middle, which means that the coupling secondary guides have rectangular cross-sections and when the smaller sides of these cross-sections are shorter than half theirlonger sides.

In certain cases, the main guide in which the lower band waves propagate must have a square cross-section such as when two systems of waves with perpendicular polarization directions are to be used simultaneously. In those cases, experience shows that transmission of the upper band through the coupling system is not properly efiected, as higher propagation modes appear in the square cross-section guide for the considered frequency band. In order to obviate these disadvantages, various, methods can be used, either each one separately or all simultaneously, as described hereunder:

(a) The coupled main andsecondary guides may be provided with plates of dielectric material, the planes of which are parallel to the electric field, which concentrates the field lines in the neighborhood of the opening of the coupling guides The electromagnetic power stored in the dielectric plates'then increases with the frequency of the wave, which allows the improving of the coupling between the main and secondary guides and the coupling guides for the sole upper band.

(b) The main guide may be coupled to two rectangular secondary guides, the smaller sides of the cross-sections of which are equal to half their longer sides; coupling is then effected by several guides. The assembly thus formed has a plane of geometric symmetry passing through one of the median lines of the cross-section of the square cross-section guide.

When the waves propagate in such a system, if they are electrically polarized perpendicularly to the plane of geometric symmetry, the electric field remains everywhere antisymmetric with respect to that plane. Junction of the two guides of rectangular cross-section which capture the energy of the upper frequency band is then effected through elbows and .a T-junction, which retain the geometric symmetry of the system as well as the antisymmetry of the electric field.

Owing to the required symmetry conditions, the junction piece for the two guides should thus be an E-type T, i. e. the plane of which is parallel to the electric field 'in the threebranches of the T.

According to another embodiment of the invention, it is also possible, as it will be shown hereinafter, to group two systems similar to that above-described, in order to independently collect the energy of two systems of waves respectively polarized perpendicularly to the two sides of the square cross-section guide.

The invention will be next described in detail in connection with the annexed drawings in which:

Fig. 1 shows the transversal section of a coupling device with a rectangular input guide in accordance with the invention, through line l-1 of Fig. 2;

Fig. 2 shows a longitudinal section of this device through line 2--2 of Fig. 1;

Fig. 3 shows a longitudinal section of this device through line 3-3 of Fig. l;

Fig. 4 shows. a first variant of the coupling device, in which theinputguide has a square cross-section;

Fig. 5 shows a second variant of the coupling device of Fig. 4.

For reducing the volume of this disclosure, the simplest embodiment of the invention, i. e. that which makes use of a main guide and of only one secondary guide, has not been shown in the annexed drawings. However, this embodiment may be constituted by half of the more complex system of Fig. 4 obtained by cutting the latter along plane 20, 21, 22. The details of the so obtained system are shown in Figs. 1, 2 and 3.

Referring to Figs. 1, 2 and 3, the two rectangular main and secondary guides 1 and 2, respectively, propagate waves according to the TE mode. Guide 1 is the input wave guide. The two guides 1 and 2 have rectangular cross-sections, the normally shorter sides of which are at most equal to their longer sides.

The waves of the upper band, the polarization direction of which is shown by the arrow 3, pass as a whole from guide 1 to guide 2.

The main guides 1 and 2 have the same cut-ofii frequency, which is lower than the lower limit of the lower frequency band to be separated. Considering the above mentioned values 35,000 and 7-10,000 megacycles per second for the limits of the bands, it is possible, for instance, to take 2,700 mc./s. as the cut-oif frequency of the guides 1 and 2.

The guides 1 and 2 are connected by the coupling guides 4 and 5 which are to be seen in longitudinal section in Figs. 1 and 2 and in transversal section in Fig. 3. The coupling guides have a common cut-off frequency comprised between the inner limits of the frequency bands to be separated and may be, for instance, equal to 6,200 megacycles per second in the example described.

The panes of the longer sides of the coupling guides 4 and 5 cut the direction of the longitudinal currents caused in the walls by the waves TE propagated in the guides 1 and 2. Therefore, only waves having frequencies pertaining to the upper band can transmit electromagnetic energy through the coupling guides 4 and 5, owing to the choice of their cut-off frequencies. The particular coupling guides 4 situated close to the ends of the coupling length of the main guide have their shorter sides smaller than the shorter sides of guides 5, situated near the middle of this coupling length.

The spacing between the successive coupling guides is approximately equal to one quarter of the phase wave length in the guide 1 for a wave having the middle fre quency of the upper band. Thus the coupling is directional owing to the fact that the partial waves transmitted through the various coupling guides are in phase in the guide 2 for a certain direction of propogation in that guide and in phase opposition for the other propagation direction. In the described example the spacing of the coupling guides is chosen to be slightly less than one centimeter.

For the lower band, the coupling guides cannot trans fer any energy. At the input of these guides there is just found a rapidly damped evanescent wave.

Assuming, for purposes of explanation of the phenomena, that a guide presents a certain analogy with a two-wire transmission line, the reactive energy contained in the wave propagated in the guide may be considered as localized in small inductances in series in the conductors of the two-wire line equivalent to the guide. These inductances could, for instance, be spaced by an inductance per unit length of the line thus obtained results in an increase of its characteristic impedance, i. e.

the characteristic impedance of guides 1 and 2.

As already mentioned, the guides 4 situated near to theends of the coupling length have the same cut-off frequency .as guides 5 situated near the middle of this length; .but'they have a smaller thickness, so, that they contain less reactive energy and consequently their characteristic impedance is intermediate that of guides 1 and 2 loaded by the coupling guides 5 and that of the unloaded guides 1 and 2. Thus, owing to the'progressive variationof the characteristic impedance, transmission without reflection of the waves of the lower band through guide 1 is obtained.

The waves of the upper band pass through the coupling guides from guide 1 to guide 2, or in the opposite direction. The optimum number of the coupling guides can be determined by experimentation. The efficiency of the coupling increases. with the number of coupling guides, reaches unity value and thereafter decreases when this number is further increased.

According to tests of constructed filters, this number can be from twelve to sixteen for coupling guides of approximately five millimeters. in thickness and of a length substantially equal to an odd number of quarter wavelengths for the average. frequency of the upper band. The tests have been particularly conclusive when both main and secondary guides have the same rectangular section with the smaller side of their cross-sections approximately equal tohalf their longer side.

As already mentioned above, the results are obviously worse when the main and secondary guides have a square cross-section. Experience shows that the upper band is then transmitted with an attenuation varying with the frequency.

However, good transmission of the upper band can still be obtained through the concentrating of the electromagnetic energy in the neighborhood of the openings of the coupling guides.

For the purpose, plates 6 and 7 of" dielectric material are put inside the guides 1 and 2, in their median planes and parallel to the direction of the electric polarization of the waves.

These plates, the cross-section of which is shown in Fig. l and the profile in Fig. 2 are all equally thick in the immediate neighborhood of the coupling guide openings; their thickness diminishes as they are more distant from the openings.

It is, known that when a dielectric plate is put in the median plane of a guide and parallel to the electric field, this field becomes much more intense in the region of the plate and diminishes with the distance separating it from the plate and this effect is more rapid, the smaller the wavelength is compared with the thickness of the plate multiplied by the factor (\/el), where e is the dielectric constant of the plate expressed in electrostatic c. g. s. units. From this it results that the energy concentration in the plate is much higher in the upper frequency band than in the lower band. This fact is very favorable, since it is the energy of the waves of the upper band which must be concentrated in the neighborhood of the openings of the coupling guides, while, on the contrary the propagation of the lower band waves should be altered as little as possible.

Further, the plate, which is thicker in the neighborhood of the openings, concentrates more energy in the corresponding region.

In order to perfect the matching, the dielectric plates may have tapered ends 8 and 9 as shown in Fig. 2. Practically, this concentration device for electromagnetic energy functions well, but a fine adjustment may be used. Therefore, it is preferable to use the filtering system shown in Fig. 4.

In this figure, the device includes an input section 10 of a main guide of square cross-section, rectangular crosssection secondary guides 11 and 12 arranged on both sides of the square guide 10. They preferably have their longer side equal to that of the square guide and their shorter side equal to half that of the square guide. The

guide 11 is ctn plecl to "section 10 by several rectangular guides 17. Likewise guide 12 is coupled to section 10 by several rectangular guides 18. i

e The waves of the upper band enter through section 10. For those waves which are electrically polarized in v a direction parallel to the. arrow 13, the electromagnetic energy passes into guides 11 and 12 and is then collected by the T-junction 14 through the intermediary of the elbows 15 and 16 respectively connected -to the guides 11 and 12. The plane of the axes. of the three branches of this T-junction is parallel to the electric field; it is, therefore, a T of the type designated E-plane. The unused ends of guides 11 and, 12, seen in Fig. 4 at the upper part of these, guides, are preferablyclosed on matched dissipating terminations so as to avoid any reflection.

The coupling guides 17' and 18 present the same construction particularities as the coupling guides 4 and 5 considered in connection withFigs. l, 2 and 3; their common cut-off frequency is chosen between the inner limits of the frequency bands to be separated.

The plane passing through points 20, 21, 22 placed in the middle plane of the guide of square cross-section is a plane of geometric symmetry for the whole of the guides, the elbows and the T-junction. A wave of the type TE entering the system of Fig. 4 and polarized in the direction of the arrow 13 is antisymmetric with respect tothe plane of geometric symmetry. The antisymmetry will necessarily be kept in the course of propagation. Therefore, whatever the wave frequency may i be, the electrical field at all points of the plane 20, 21,

22 is perpendicular to. that plane. It is, therefore, possible to consider that, without: disturbing the propagation, everything goes on as if a metallized perfectly conductive plane was placed in the considered symmetry plane and thus to divide the system shown in Fig. 4 into two systems, identical from the electrical as well as from the geometrical viewpoint, to that shown in Figs. 1, 2 and 3, in the case where the two main guides would have the same rectangular cross-section, with their shorter side approximately half as long as their longer side.

Waves that are polarized perpendicularly to the arrow 13 do not transmit energy to the coupling guides 17 and 18, whatever their frequencies may be, as for these waves the openings of these guides appear like slots parallel to the current lines.

In order to separate the latter waves, without disturbing the waves polarized in a direction parallel to arrow 13, coupling guides must be used, the longer side of which is perpendicular to line 2122. Thus the device of Fig. 5 is obtained.

In this figure the reference numbers 10 to 22 relate to the same elements as those elements designated by the same numbers in Fig. 4. As mentioned for this last figure, the coupling guides 17 and 18 collect the wave energy of the upper band polarized in the direction of arrow 13 and direct it towards the E-plane type T- junction 19 through the intermediary of guides 25 and 26.

What is claimed is:

1. A band separating filter adapted to be inserted in a main wave guide of rectangular cross-section in which two signal groups, the frequencies of which respectively cover a lower frequency band and an upper frequency band separated by a frequency interval, propagate according to the TE mode, said filter comprising a length of said main guide at the input of which said two signal groups are applied and at the output of which the signals of said lower band are received at least one secondary wave guide receiving the signals of said upper band and being of rectangular cross-section, said secondary guide having its axis parallel to that of said main guide and being coupled to the latter said guide by a first series of slots distributed along said length of said main guide and provided in its walls and by a second series of slots provided in the walls of the longer sides of said secondary guide or, guides, and coupling assesses wave guides, the slots of said first and secondary series being connected in pairs by said coupling wave guides, said coupling wave guides having a rectangular cross-section equal to the surface of both of the slots to which they are connected and axes which are perpendicular to those of said main and secondary guides, said coupling guides having equal cutoff frequencies of values chosen between, the highest frequency of said lower frequency band and the lowest frequency of said upper frequency band, the dimensions 1 of the cross-sections of said coupling guides varying progressively along the length of said main guide so that their characteristicimpedances have a maximum value in the middle and a minimum value at the ends of said length, the length of said coupling, guides measured along their axes being substantially equal to a quarter phase wavelength at the middle frequency of said upper frequency band, and the spacing between the axes of two successive of said coupling guides being substantially equal to a quarter phase wavelengthin said main guide of said middlefrequency.

2. A filter as claimed in claim 1 comprising a T-junction and an even number of secondary guides and wherein said secondary guides are grouped in pairs and the two guides of a same pairare coupled to a common utilization circuit by said T-junction; said T-junction comprising three lengths of rectangular cross-section guides, with the plane containing the three axes of said guide lengths beingparallel to the direction of the electric field in the secondary guides of said pair.

3. A filter as claimed in claim 1 comprising one main and one secondary guide wherein the cross-sections of latter said guides are equal between themselves and wherein for each one of said cross-sections, the length of its longer side is substantially twice that of its shorter side.

4. A filter as claimed in claim 1 comprising a main guide of square crosssection and two secondary guides with equal rectangular cross-sections, the longer side of said cross-sections being equal in length to that of said square cross-section, the shorter side of said cross-see tions of said secondary guides having a length substantially equal to half that'of their longer, side.

5. A filter as claimed in claim 1 comprising a main guide of square cross-section and four secondary guides of equal rectangular cross-sections, the longer side of latter said cross-section being equal in length to that of said square cross-section, the shorter side of said crosssection of said secondary guides having a length substantially equal to half that of their longer side.

6. A filter as claimed in claim 1 wherein plates of dielectric material are set up in said main and secondary guides in planes parallel to the electric field in said guide and substantially in front of the centers of the cross-sections of said coupling guides.

7. A filter as claimed in claim 6, wherein said plates have a maximum thickness at those of their ends which are nearest to the openings of said coupling guides into said main guide and a minimum thickness at those of their ends which are farthest from said openings.

References Cited in the file of this patent 

